1. Field of the Invention
The present invention relates to an active material for a battery, a non-aqueous electrolyte battery and a battery pack.
2. Description of the Related Art
Non-aqueous electrolyte batteries which charge and discharge by transfer of lithium ions from the negative electrode to the positive electrode and vice versa are studied and developed enthusiastically.
These non-aqueous electrolyte batteries are desired to have various characteristics according to each use. They are most likely used, for example, in a about 3 C discharge operation for a power source of a digital camera and a about 10 C discharge operation for automobiles such as hybrid electric cars. For this reason, the non-aqueous electrolyte batteries for these uses are desired to have large-current characteristics in particular.
At present, non-aqueous electrolyte batteries using a lithium-transition metal composite oxide as the positive electrode active material and a carbonaceous material as the negative electrode active material are commercially available. In these lithium-transition metal composite oxides, Co, Mn and Ni are generally used as the transition metal.
In these days, much attention has been focused on a lithium-titanium composite oxide having a higher lithium absorption and releasing potential than a carbonaceous material (see, for example, JP-A 10-247496 (KOKAI)). The lithium-titanium composite oxide has the advantage that metal lithium does not precipitate at this lithium absorption and releasing potential in principle and is superior in rapid charging and low-temperature performance.
Among these lithium-titanium composite oxides, lithium titanate having a spinel structure as described in JP-A 8-22841 (KOKAI) attracts considerable attention because it has only small change in volume in a charge and discharge operation and also has high reversibility.
With regard to a non-aqueous electrolyte battery using a carbonaceous material as the negative electrode active material, the lithium absorption/releasing potential of the carbonaceous material is as low as about 0.1 V vs. Li/Li+, and therefore, a stable coating film called SEI (solid electrolyte interface) is formed on a surface of the negative electrode, thereby limiting the decomposition of the non-aqueous electrolyte (for example, a non-aqueous electrolyte solution) on the surface of the negative electrode. On the other hand, the lithium absorption/releasing potential of the above lithium-titanium composite oxide is as high as about 1 to 2 V vs. Li/Li+, and therefore, it is difficult to form a stable coating film on its surface, whereby the decomposition of the non-aqueous electrolyte solution proceeds continuously. Particularly, when a positive electrode provided with an active material containing Mn is combined with this negative electrode, Mn ions eluted from the Mn-containing active material act on the negative electrode and decompose the non-aqueous electrolyte solution significantly. As a result, gas is generated in the battery, which causes swelling of the battery.